Ultrafast Band-Gap Shift Induced by a Strain Pulse in Semiconductor Heterostructures

The conventional piezospectroscopic effect is extended to picosecond time scales by using ultrashort strain pulses injected into semiconductor heterostructures. The strain pulses with durations of $\sim 10\mathrm{ps}$ are generated in a metal transducer film by intense femtosecond laser pulses. They propagate coherently in the $\mathrm{GaAs}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure over a distance of $100\mu \mathrm{m}$ and shift the band gaps by several meV as detected optically for quantum well exciton resonances by pump-probe techniques and time-resolved photoluminescence.

Figure 1

(a) Sample, (b) experimental scheme, and (c) reflectivity spectra measured at different delay times $\Delta t=t-t_0$ ($t_0\approx \mathrm{ns}$) for $W=3\mathrm{mJ}/{\mathrm{cm}}^2$. The vertical arrows in (c) indicate the energy of the exciton resonance. The solid lines show smoothed curves.

Figure 2

(a) Measured (dots) and calculated (solid line) temporal evolution of the dynamical band-gap shift $\Delta E(t)$ induced by strain pulses in the 10 nm $\mathrm{GaAs}/{\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ QW. The vertical arrows show the arrival of incident and expected reflected strain pulses. (b) Calculated temporal evolution of initial strain pulse ${\epsilon }_0(t)$ injected from the metal transducer into the GaAs substrate. The inset shows schematically the incident and reflected strain pulses acting on the QW energy levels.

Figure 3

Phonon-induced dynamics in a 10 nm $\mathrm{GaAs}/{\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ QW: (a) Reflectivity $\Delta R(t)/R$ and Kerr rotation $\theta (t)$ signals measured in pump-probe experiments without spectral resolution at $T=10\mathrm{K}$ and $W=5\mathrm{mJ}/{\mathrm{cm}}^2$. The inset shows the power dependence of the mean intensity for $\Delta R(t)/R$ (open symbols) and $\theta (t)$ (solid symbols) in the spike part of the signal (vertical dashed lines show the integration interval from 0 to 80 ps). (b) Kerr rotation signals measured at different temperatures for $W=5\mathrm{mJ}/{\mathrm{cm}}^2$. The vertical arrows indicate the arrival time of a strain pulse in a 10 nm QW. The inset shows the measured temperature dependence of sound velocity in GaAs.

Figure 4

(a) cw PL spectrum of the studied $\mathrm{GaAs}/{\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ QW heterostructure. Temporal evolution of normalized peak PL intensity in (b) 20 and (c) 5 nm QWs induced by ultrafast strain pulses.